Add warning about aks-preview extension incompatibility with AKS Automatic BYO VNet

[pineapplethevoyager]

Summary

Adds an IMPORTANT callout box to both AKS Automatic custom VNet quickstart docs (private and public) explaining that the aks-preview CLI extension is incompatible with these deployments.

Context

Since hosted system pool GA on June 2, 2026, the aks-preview extension overrides the az aks create command surface and hides the required --node-subnet-id and --system-node-subnet-id parameters. Customers who have the extension installed and fall back to the legacy --vnet-subnet-id parameter will have their deployment accepted by the RP but it will fail after up to 2 hours during node provisioning (NIC creation against a non-existent VNet).

The existing prerequisite bullet says to uninstall the extension but does not explain the consequences of ignoring it. This PR adds a prominent warning callout that explains:

• What happens if you leave it installed
• Why the deployment fails silently (accepted but times out)
• That you must always remove it before creating AKS Automatic clusters with custom VNets

Related

• ICM 51000001094157
• Affects: articles/aks/automatic/quick-automatic-private-custom-network.md
• Affects: articles/aks/automatic/quick-automatic-custom-network.md

[prmerger-automator]

@pineapplethevoyager : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[prmerger-automator]

@pineapplethevoyager : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[v-dirichards]

Hi @pineapplethevoyager - This pull request was opened in the public repo. Microsoft contributors should work in the private repo, per the Microsoft Docs contributor guide. We can keep this PR open for review and merge, but would you make future content updates in the private repo? Thank you!

---

@wangyira

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[learn-build-service-prod]

Learn Build status updates of commit a815f00:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/aks/automatic/quick-automatic-custom-network.md	✅Succeeded
articles/aks/automatic/quick-automatic-private-custom-network.md	✅Succeeded

For more details, please refer to the build report.

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